Electro-pyrotechnic initiator, method for making same, and vehicle safety system

ABSTRACT

An electro-pyrotechnic initiator includes two electrical pins (7) for supplying current, a pyrotechnic composition (4) stored in a receptacle (2, 3) and a bridge (23) establishing an electrical connection between the two pins (7). The bridge (23) is set down on a wafer (11), the bridge (23) and the wafer (11) forming part of an electronic micro-component (20) to which the pins (7) are connected. The composition (4) is compacted in the receptacle (2, 3), the composition (4) and the micro-component (20) being brought together under pressure in the receptacle (2, 3). A housing for a helium saturated material is provided at a bottom of the receptacle.

This invention relates to an electro-pyrotechnic initiator.

The purpose of this type of device is to be ignited by the passage of anelectric current greater than a predetermined threshold value, so as toput a mechanism into action.

The electro-pyrotechnic initiators are intended to be integrated intoelectro-technical chains. In particular, they are used in safety systemsfor a car or for other vehicles, for example, in order to activate asafety air-bag. They are also used, for example, in missiles.

Such an electro-pyrotechnic initiator is known, for example, fromdocument U.S. Pat. No. 5,230,287. This known electro-pyrotechnicinitiator comprises two electrical pins for supplying current, insertedinto a baseplate, a bridge establishing an electrical connection betweenthe two pins, a pyrotechnic composition stored in a receptacle, anelectronic circuit comprising a bridge laid down on a wafer, this waferbeing connected to the electrical pins for supplying current, and thepyrotechnic composition being compacted in the receptacle and broughttogether with the micro-component under pressure in the receptacle.

The invention also relates to a method of producing anelectro-pyrotechnic initiator, and a safety system for a vehiclecomprising one or several initiators.

The initiators used until now have usually consisted of a devicecomprising two electrical pins, connected by a soldered filament thatforms a conducting bridge arranged in contact with a pyrotechniccomposition.

The rudimentary aspect of the connection between the pins has the majordisadvantage of not providing sufficient control over the ignition.

In more highly developed initiators, the conducting bridge is made on aprinted circuit or an equivalent support. Greater precision is thenpossible.

However major disadvantages remain. In particular, a bead of solder isnecessary at the electrical connection between the conducting bridge andthe pins for supplying current. This impairs the flatness of theinterface of the conducting bridge with the pyrotechnic composition andaffects the evenness of the heat exchange. The bead of solder alsonecessitates to use deposition technology, involving the deposition ofdrops or of pyrotechnic product paint, technology which is not highlyregarded for large scale serial manufacture.

Furthermore, these devices have an architecture which does not equallywell provide for receiving a conducting or a semi-conducting bridge.

Another disadvantage of the initiators usually used is that protectionagainst discharges of static electricity is not taken into account. Veryserious consequences can result to the user from this.

Regarding the detection of leaks, usually it is done by the injection ofhelium and then detection through aspiration. This method is timeconsuming to implement and is not very reliable.

The invention is directed towards remedying these disadvantages.

The aim of the invention is thus an electro-pyrotechnic initiator thatpermits precise control of an ignition, that is capable of beingintegrated into a high performance chain calling for a high level ofreliability.

Another object of the invention is an initiator capable of equally wellreceiving a conducting or a semi-conducting bridge, with identicalarchitectures, the semi-conducting bridge providing a more rapid heatexchange with the pyrotechnic composition.

It is also an aim of the invention to provide an initiator that isstable over time.

A further object of the invention is to provide protection againstdischarges of static electricity.

A supplementary aim of the invention is a technology that makes use of acompacted form of the pyrotechnic composition, that allows the integrityof the interface between the bridge and the composition to be preservedeven after long storage.

Another object of the invention is an initiator having a long life dueto its gas tight properties.

The invention also has as an object, a very simple method of producingan electro-pyrotechnic initiator having the advantages above.

The invention relates also to a safety system for a vehicle thatcomprises such an initiator.

To this effect, the invention relates to an electro-pyrotechnicinitiator comprising;

two electrical pins for supplying current inserted into a baseplate,

a bridge setting up an electrical connection between the two pins,

a pyrotechnic composition stored in a receptacle.

According to the invention, this bridge is set down on a wafer, thebridge and the wafer forming part of an electronic micro-component, thewafer is connected to the pins and the composition is compacted in thereceptacle. The composition and the micro-component are brought togetherunder pressure in the receptacle.

In comparison with existing initiators, the pyrotechnic initiatoraccording to the invention thus provides a unique architecture thatallows the integration of a conducting or a semi-conducting bridgeequally well. What is more, the initiator can be produced from twosub-assemblies, respectively an electrical and a pyrotechnicsub-assembly, assembled under pressure in such a way that the integrityof the interface between the micro-component and the composition ispreserved. The compaction of the pyrotechnic composition permits, inaddition, perfect control of the ignition thanks to the bridge of theelectronic component.

According to one preferred embodiment of the initiator, the receptaclehas gas tight walls, the baseplate is a gas tight crossover and theconnection between the walls and the baseplate is also gas tight. Henceinfiltration into the composition is avoided.

The initiator advantageously comprises a housing inside the receptacle,this housing comprising a material saturated with helium, allowing thedetection of possible leaks.

Instead of the laborious method usually used to detect a possible leak,the initiator according to the invention permits very simple testing ofgas tightness. The material saturated with helium, in effect constitutesa sponge and any leakage of the case causes a loss of helium that can beeasily detected by aspiration.

In the preferred embodiment, with gas tight walls, it is judicious forthe receptacle to comprise an electrically insulating cup and a metalcase with gas tight walls containing this cup.

Preferably the receptacle has one area of these walls that has beenthinned or weakened in such a way that ignition of the pyrotechniccomposition has a favoured effect into this area.

Advantageously, the initiator comprises an insulating cap which, withthe cup, constitutes an electrically insulating cage protecting thepyrotechnic composition from any attack resulting, in particular, fromdischarges of static electricity.

In a first configuration of the initiator, the bridge, being aconductor, is constituted by a resistive layer of constant thickness.

The resistive layer advantageously has a rectangular surface, forexample, a square surface.

An accurate determination of the resistance of the component and of asize of current capable of triggering the initiator is hence obtained.

In a second configuration of the initiator according to the invention,the bridge is a semi-conductor.

In addition, two contact pads are advantageously provided at oppositeends of the wafer, these pads being intended to facilitate the brazingof the pins.

This device eliminates any problem posed by beads of solder.

The invention also relates to a method of producing anelectro-pyrotechnic initiator. According to the invention:

in a first step, a pyrotechnic composition is compacted in a receptacle,in such a way as to form a pyrotechnic sub-assembly,

in a second step, a bridge is set down on a wafer and this wafer isbrazed onto electrical pins embedded in a baseplate in such a way as toform an inert sub-assembly,

in a third step, the pyrotechnic and the inert sub-assemblies are joinedtogether under pressure.

This separate assembly of the two sub-assemblies provides at the sametime simplicity of implementation and great precision in the triggeringcharacteristics.

The invention also relates to a safety system for a vehicle including atleast one electro-pyrotechnic initiator according to the invention,liable of activating a safety mechanism.

The invention will be described in greater detail, making reference tothe drawings, in which:

FIG. 1 represents a longitudinal section of an inert sub-assembly,comprising electrical elements, forming part of an initiator accordingto the invention.

FIG. 2 represents a longitudinal section of a pyrotechnic sub-assemblycorresponding to the inert sub-assembly in FIG. 1.

FIG. 3 shows an assembly of the sub-assemblies respectively the inertand the pyrotechnic sub-assemblies of FIGS. 1 and 2, constituting theinitiator described by way of example.

FIG. 4 represents an electronic micro-component used in the initiator ofFIG. 3, along a cross section IV--IV.

FIG. 5 represents a perspective view of the bridge in FIG. 4.

The electro-pyrotechnic initiator 1, representative of the invention,and shown in FIG. 3, comprises two parts: a first inert sub-assembly 18comprising, electrical elements and a second pyrotechnic sub-assembly19, containing products used for ignition.

The inert sub-assembly 18, represented in FIG. 1, comprises twoelectrical pins 7 intended to be connected to an electrical circuit.These two metal pins 7 for supplying current are typically made of FN50iron nickel alloy. Their diameter is, for example 1 mm. The pins arehermetically sealed into an electrically insulating baseplate. This ispreferably made of glass, but can equally well be made of ceramic orplastic, for example. A cylindrical body 9 surrounds this baseplate 8 towhich it is hermetically joined. This metal body 9 is preferably made instainless steel. The baseplate 8 is covered with a cap slipped onto thepins 7. This cap 10 is made of an electrically insulating material, andis preferably made of plastic. Typically it is made in a polyamide. Thecap 10 electrically insulates the metal body 9 from the pyrotechnicpart.

An electronic micro-component 20 is brazed onto the pins 7. The cap 10makes its positioning easier and contributes to the insulation of thepyrotechnic part.

The pyrotechnic sub-assembly 19, that can be seen in FIG. 2, comprisesthe initiation pyrotechnic composition 4. This is a substance sensitiveto heat. Typically it can be chosen from among lead azide, tetrazene,the lead mononitro-resorcinates, the lead dinitro-resorcinates and thelead trinitro-resorcinates. The latter substance is the one chosen inthe example.

The pyrotechnic composition 4 is accompanied by a composition 5 thatreinforces the pyrotechnic effect. This reinforcing composition 5 is ofthe oxidoreduction type. Its reducing agent is typically based onzirconium, boron, titanium, or titanium or boron hydride. In a preferredembodiment. It contains 50 to 70% potassium perchlorate, 0 to 10% ofboron, 20 to 40% titanium hydride and 2 to 5% fluorinated binding agent.

The compositions 4 and 5 are compacted under pressure in an insulatingcup 3. This is preferably made of plastic material. The composition 5for reinforcing the pyrotechnic effect is compacted towards the bottom16 of the cup 3, and the pyrotechnic composition 4 is compacted aboveit. For example, 50 mg of composition 5 and then 30 mg of pyrotechniccomposition 4 are compacted successively.

The cup 3 containing the pyrotechnic composition 4 and the composition 5for reinforcing the pyrotechnic effect is itself contained in a case 2,preferably made of metal.

The cup 3 and the cap 10, when assembled constitute an electricallyinsulating cage which protects the pyrotechnic composition from anyattack, resulting, in particular from discharges of static electricity.

The case 2 has a bottom 15 which has been thinned in such a way as tocreate a preliminary weakening of it. An ignition of the pyrotechniccomposition 4 thus has a favored effect in the direction of the bottom15. The bottom 16 of the cup 3 also has a reduced thickness.

Between the bottom 16 of the cup 3 and the bottom 15 of the case 2, ahousing 6 is provided. This housing 6 is filled with a materialsaturated with helium, preferably being in the form of a thin disc. Thisis set down in the bottom 15 of the case 2 before putting in the cup 3.This device allows the hermetic sealing of the initiator 1 to be testedeasily with the help of a helium detector.

The inert sub-assembly 18 and the pyrotechnic sub-assembly 19 areassembled in compression around a common axis 13, as shown in FIG. 3 toform the initiator 1. The case 2 and the body 9 are made integral by alaser ring weld 12. This weld permits the maintenance of constantcompression and guarantees the hermetic seal. By way of example, it canguarantee a compressive condition of 500 bars to a pyrotechnic charge.The assembly of the two sub-assemblies 18, 19 can also be carried out bysticking the case 2 and the body 9 together.

The assembly under pressure of the two sub-assemblies 18, 19 preventsthe presence of air between the micro-component 20 and the pyrotechniccomposition 4 which would impair control of the heat exchange. Thegeneral advantage of this assembly is that it allows the integrity ofthe interface between the bridge 23 and the composition 4 to bepreserved, this being created precisely in accordance with the desiredconfiguration.

In the assembly thus developed, the cap 10 electrically insulates thepyrotechnic composition 4 from the metal body 9. The glass baseplate 8provides for itself the hermetic sealing of the passage between the pins7 and the metal body 9 and also allows the compressive condition to bemaintained.

The weld 12 prevents infiltration between the case 2 on the one hand andthe body 9 and the cup 3 on the other hand, thereby providing anexcellent hermetic seal for the contents of compositions 4 and 5. Italso permits maintenance of the compressive condition.

The electronic micro-component 20, detailed in FIG. 4, comprises a wafer11, preferably made of alumina. By way of example, it has a length L1 of3 mm in the direction moving from one pin 7 to the other, a width L2equal to 2.54 mm and a thickness of 0.635 mm. On this wafer 11, using aphoto-lithography method, two conductive tracks 22 are laid down on orsymmetrically arranged at the side of each of the pins 7. The tracks 22are separated by a gap L3 of about 100 μm.

A bridge 23 is serigraphed onto this gap L3. It can also bephotolithographed or deposited under vacuum. In the example presented,it is a resistance, this typically being created by serigraphy with thehelp of ruthenium based ink.

The bridge 23, visible in FIGS. 4 and 5, has a length L'1 in accordancewith the length L1 of the wafer 11, a width L'2 in accordance with thewidth L2 of the wafer 11 and a thickness e. Hence it has a surface areaS equal to L'1×L'2. Also its cross section is defined as e×L'2. Bycalling p the resistivity of the bridge 23, the resistance R of thebridge is given by: ##EQU1##

If one keeps a constant ratio of L'1/L'2 for the rectangular surface Sof the resistive bridge 23, the resistance R is simply modified byvarying the thickness e and the resistivity ρ. In particular, it ispossible to choose a square surface S, in which case L'1 is equal toL'2. In this case, the resistance R has the value ρ/ e.

The choice of lengths L'1 and L'2 permits adjustment of two importantvalues: a non-operational current I0 and an operational current I1. Thecurrent I0 defines a current threshold passing through the bridge 23short of which the heating of the wafer 11 is not sufficient to triggerignition of the initiator 1. The operational current I1, greater thanthe non-operational current I0, defines a threshold beyond which theinitiator 1 is systematically ignited. Between I0 and I1, triggering ofthe initiation is not guaranteed.

The currents I0 and I1 can be easily adjusted by taking action on thesurface area S: the smaller it is, the lower the values of I0 and I1.The composition of the bridge 23 also plays a pail in theirdetermination.

By carefully choosing the dimensional, mechanical and thermalcharacteristics of the resistive bridge 23, it is therefore possible toadjust precisely the resistance R and the currents I0 and I1.

In the example chosen, the bridge 23 has a square surfaces, and thelengths L'1 and L'2 are both equal to 150 μm. For an automobile, theresistance R typically has a value of the order of 2Ω±0.15Ω.

The contact pads 21 made of platinum-gold are also deposited at oppositeends of the wafer 11 corresponding to the positioning of the pills 7.The pads 21 cover these ends over the whole width L2 of the wafer 11.These deposits are made on both faces of the wafer 11 as well as on theedges 24 corresponding to the ends involved. The pads 21 facilitate thebrazing of the pills 7, in the same way as for electronic componentsmounted in surface onto printed circuits. The pins 7 are of the kindconnected to the wafer 11 in accordance with a widely proven solderingmethod.

The assembly comprising the wafer 11, the resistive bridge 23, theconductive tracks 22, and the contact pads 2 1 constitutes theelectronic micro-component 20.

In the case of a semi-conducting bridge 23 the architecture adopted isexactly the same. Typically, the wafer 11 is then made of asemi-conductor material deposited onto a support made of a ceramicmaterial.

The use of a semi-conducting bridge 23 is particularly advantageous,since it allows great precision in the triggering of the initiator 1. Ineffect, it only becomes resistive at a fixed level of voltage. When thislevel is reached, it causes a cascade phenomenon, beginning with aheating up of the bridge 23, going hand in hand with a reduction in itsresistance and ending with the discharge of a plasma in the composition4. A minimum duration of application of the current is required to causean ignition.

While the thermal exchanges for a resistive component 20 are broughtabout through conduction, they occur essentially through convection inthe semi-conductor case. Although the importance of a perfectlycontrolled interface between the composition 4 and the component 20 isrecognised to be not as great, it remains desirable.

During the assembly, the inert 18 and pyrotechnic 19 sub-assemblies areassembled under pressure, a pressure, for example, of 500 bars beingexerted on the pyrotechnic composition 4. They are subsequently joinedby the weld 12, which maintains the pressure applied beforehand afterrelease.

The architecture is independent of the way in which the sub-assemblies18 and 19 are produced. This simplifies the manufacture of the two typesof initiator on the same site from sub-assemblies 18, 19. This generalconfiguration makes it possible to obtain great precision in theignition.

The gas tightness of the operational initiator 1 is subsequentlyverified before its installation in a vehicle with a helium detector, inorder to detect any possible leaks coming from the material saturatedwith helium stored in the housing 6.

In operation, in the presence of a conducting bridge 23, a small currentpasses through the pins 7 connected to an electrical circuit in thecomponent 20. This current, which can be zero, is insufficient totrigger the initiator 1. It is less than the non-operational current 10for the conducting bridge 23. Ignition of the initiator 1 is caused byan appreciable increase in the current passing through the pins 7 andgreater than the operational current I1. Very rapid heating of theconducting bridge 23 then ensues, which is transmitted to thepyrotechnic composition 4 through the wafer I1. An explosion is thentriggered, its effects being propagated in a favoured way in thedirection of the bottom 16 of the cup 3.

If the bridge 23 is a semi-conductor, the operation is entirely similar.Ignition of the initiator 1 is however caused in a more abrupt fashionas soon as a current passing through the pins 7 has a value exceeding athreshold value associated with the semi-conducting bridge 23 and itsduration is sufficient.

Although the bridges 23 presented by way of example are particularlyadvantageous, other configurations can also be envisaged. In particular,the bridge 23 can be constituted by a thin layer as well as by a thicklayer. It can also have a surface S that is other than rectangular andis, for example of a circular or polyhedral shape.

Furthermore, if the gas tightness is not as good as in the exampledescribed and there would be creep modifying the interface between thecomponent 20 and the pyrotechnic composition 4, it could be contemplatedto produce the case 2 and the body 9 in a plastic material. The presenceof the insulating cup 3 would then no longer be necessary.

The initiator according to the invention is usable, in particular, invehicles to put safety mechanisms, such as air bags, devices for lockingand unlocking doors or pretensioning devices for safety belts, intoaction rapidly. However, it is also suitable for any other device thatrequires rapid triggering and good control over a mechanism. By way ofexample, it is usable in offensive or defensive military systems and insystems for protection fires or floods.

We claim:
 1. An electro-pyrotechnic initiator (1) comprising:twoelectrical pins (7) for supplying current inserted into a baseplate (8),a bridge (23) setting up an electrical connection between the two pins(7), a pyrotechnic composition (4) stored in a receptacle (2, 3)comprising an electrically insulating cup (3) and a metal case (2) withgas tight walls containing said cup (3), said pins extending from a topof said case, said bridge (23) being laid down on a wafer (11), thebridge (23) and the wafer (11) forming part of an electronicmicro-component (20) and the wafer (11) being connected to the pins (7)and the composition (4) being compacted in the receptacle (2, 3), thecomposition (4) and the micro-component (20) being brought togetherunder pressure in the receptacle (2, 3), and a housing (6) inside thereceptacle (2, 3) between a bottom (16) of said cup (3) and a bottom ofsaid case (2), said housing (6) comprising a material saturated withhelium, allowing the detection of possible leaks, by aspiration.
 2. Anelectro-pyrotechnic initiator (1) according to claim 1, wherein, thebaseplate 8 is a gas tight crossover and the connection between thebaseplate 8 and the walls of the receptacle (2, 3) is also gas tight. 3.An electro-pyrotechnic initiator according to claim 2, furthercomprising an insulating cap (10) which, with the cup (3), constitutesan electrically insulating cage protecting the pyrotechnic compositionfrom discharges of static electricity.
 4. An electro-pyrotechnicinitiator (1) according to claim 1, wherein the receptacle (2, 3) has anarea (15, 16) at the bottom of said case (2) which forms a wall of saidhousing (6) that is weakened so that igniting the pyrotechniccomposition (4) has a favored effect into said area (15, 16).
 5. Anelectro-pyrotechnic initiator (1) according to claim 1, wherein thebridge (23) comprises a resistive layer (23) of constant thickness (e).6. An electro-pyrotechnic initiator (1) according to claim 5, whereinthe resistive layer has a rectangular surface (S).
 7. Anelectro-pyrotechnic initiator (1) according to claim 1, wherein thebridge (23) is a semi-conductor.
 8. An electro-pyrotechnic initiator (1)according to claim 1, further comprising two contact pads (21) atopposite ends of the wafer (11), said pads (21) for facilitating brazingof the pins (7).